1. Field of the Invention
The invention relates to circuit breakers, and more particularly to circuit breakers including electronic circuitry for controlling an undervoltage release mechanism.
2. Description of the Prior Art
Circuit breakers are widely used in industrial, commercial, and residential applications to provide protection for electrical apparatus and distribution equipment. Upon overcurrent conditions through a connected circuit, the circuit breaker will automatically open to interrupt current flow through the circuit. Certain applications such as in the mining industry require a fail-safe undervoltage release or remote tripping capability. This requires that the circuit breaker contacts be opened whenever a control voltage falls below a predetermined minimum. In the aforementioned U.S. patent application Ser. No. 595,184 there is disclosed a circuit breaker including an undervoltage release mechanism which employs a holding coil to oppose the action of a bias spring upon a plunger, thereby preventing the tripping of the circuit breaker as long as current is applied to the holding coil. The undervoltage release mechanism also employs a trip coil which is energized upon overcurrent conditions through the circuit breaker. Energization of the trip coil will aid the action of the bias spring and overcome the action of the holding coil to operate the plunger to trip the circuit breaker.
The holding coil must generate sufficient force upon the plunger to resist the action of the bias spring and yet limit the force produced so that the trip coil may properly operate to trip the circuit breaker upon overcurrent conditions. The current through the holding coil must therefore be regulated to a fairly constant value.
The input voltage upon the control line which operates the undervoltage release mechanism can vary over a wide range. For instance, a nominal 120 volts RMS line can vary from 0 to 150 volts or more. It is desirable to energize the holding coil at a value considerably less than rated voltage, such as 50%. The control circuit for the undervoltage release mechanism must therefore operate properly from approximately 60 volts to 150 volts.
It is desirable to provide a circuit breaker with undervoltage release capability which is entirely self-contained. Thus, it is desirable that the control circuitry for the undervoltage release mechanism be mounted within the case of the circuit breaker. Since the control circuit must then operate in a confined space, it is desirable to minimize the heat generated by components of the control circuits.